Defensins, e.g., human neutrophil peptides (HNPs), contribute to innate immunity through diverse actions including microbial killing;high concentrations are present in the lung in response to inflammation. Arginines are critical for HNP activity, which is decreased by their replacement with ornithine. ADP-ribosyltransferases (ARTs) catalyze transfer of ADP-ribose from NAD to an acceptor arginine in a protein substrate, whereas ADP-ribosylarginine hydrolases release ADP-ribose. ART1 on the surface of airway epithelial cells ADP-ribosylated HNP-1 specifically on arginines 14 and 24, with ADP-ribosylation altering biological activity. Di- and mono-ADP-ribosylated HNP-1 were isolated from bronchoalveolar lavage fluid (BALF) of patients with asthma and idiopathic pulmonary fibrosis (IPF), suggesting a role for ADP-ribosylation in disease. In the present study, we observed that ART1-catalyzed ADP-ribosylation of HNP-1 in vitro generated a novel product with ADP-ribose on arginine 24, and ornithine replacing arginine at position 14. We hypothesized that ADP-ribosylarginine is susceptible to a non-enzymatic hydrolytic reaction yielding ornithine. On incubation of di- or mono-ADP-ribosyl-HNP-1 at 37 C, ADP-ribosylarginine was partially replaced by ornithine, whereas ornithine was not detected by amino acid analysis and mass spectrometry of unmodified HNP-1 incubated under the same conditions. Further, ornithine was produced from the model compound, ADP-ribosylarginine. BALF from an IPF patient contained ADP-ribosyl-HNP-ornithine as well as mono- and di-ADP-ribosylated HNP-1, consistent with in vivo conversion of arginine to ornithine. Targeted ADP-ribosylation of specific arginines by transferases, resulting in their replacement with ornithine, is a novel alternative pathway for regulation of protein function through post-translational modification.